These days, the development of an avalanche photodiode which is fabricated from In.sub.0.53 Ga.sub.0.47 As compound semiconductor has been promoted. Such an avalanche photodiode can be applied to an optical fiber transmission system in which 1 to 1.6 .mu.m wavelength region is dominantly used to transmit an optical information through an optical fiber with a low transmission loss. Since the InGaAs semiconductor is lattice-matched to a wide energy gap InP semiconductor, heterostructure comprising with the InGaAs and InP can be obtained. In the heterostructure, the InGaAs semiconductor is for a light absorption layer, and the InP layer is for an avalanche multiplication layer into which either electrons or holes generated in the light absorption layer inject to produce the avalanche multiplication. Using such separated absorption and nultiplication structure, a photo detector can be realized with an excellent receiving sensitivity.
The concept as described above has been described on pages 251 to 253 of "Appl. Phys. Lett. Vol. 35, 1979" by K. Nishida et al. An avalanche photodiode fabricated in accordance with the concept will be explained in detail later.
According to the avalanche photodiode, however, there is a disadvantage that it is difficult to provide a guard-ring effect by which an edge breakdown is prevented from being occurred at an outer peripheral edge of a p.sup.+ -conduction region selectively formed in a multiplication layer, although the reason thereof will be also explained later.